Generally, a channel estimation method in a wireless communication system is classified into a training-based channel estimation method using a preamble signal or a pilot signal, a semi-blind method additionally using a data signal after using properties of the preamble signal, the pilot signal or other predetermined signals, and a blind method using only a received data signal without predetermined signals.
If the blind method is employed, a throughput due to channel estimation is not deteriorated since additional signals are not used. However, the blind method uses properties of the received signal only, thus reducing channel estimation performance, especially in the case of rapid change in channels. Regarding the training-based method, as it uses the predetermined signals, it provides the best channel estimation performance but requires additional signals in proportion to the number of transmission antennas. Hence, it has a problem that the throughput tends to deteriorat according to the increased number of transmission antennas or according to the enhancement of estimation performance. Meanwhile, as the semi-blind method estimates a channel by using the data signal together with the preamble signal or the pilot signal, additional overhead due to the enhancement of channel estimation performance does not occur and therefore the throughput is not reduced.
An accurate channel estimation is required to detect multiple signals or to effectively remove an interference signal of a neighboring cell in a multi antenna wireless communication system or a cellular system having a frequency reuse rate of 1. In the multi antenna wireless communication system, a terminal using a plurality of reception antennas uses a channel response in order to classify signals generated by spatial multiplexing or space-time coding from a plurality of transmission antennas. In the cellular system having the frequency reuse rate of 1, a terminal at a cell boundary has a difficulty in receiving multiple signals from a serving base station due to an interference signal from a neighboring base station and the channel deterioration with the serving base station. Thus, the base station generally transmits the same data to the terminal at the cell boundary through a plurality of transmission antennas to thereby improve reliability in signal reception of the terminal. In this case, the terminal at the cell boundary determines the interference signal of the neighboring base station and the signal from the serving base station as the multiple signals transmitted by a transmission terminal employing the plurality of transmission antennas virtually and employs an interference removal technique, thereby removing the interference signal from the neighboring base station.
For example, the semi-blind method includes expectation maximization (EM) and decision directed (DD). The EM is a method of estimating a channel response which is in the closest proximity to a received data signal in probability. The DD is a method of detecting a channel response estimated by an initial pilot signal or a preamble signal and then updating the detected channel response according to a channel change by using a received data signal. That is, the DD method considers the detected signal as a transmitted signal and estimates a channel change by applying a general channel estimation method. An area which satisfies coherence time and coherence bandwidth having an equal channel response is formed as a group. If the number of data sub-carriers in the group is larger than that of pilot or preamble signals, it can be expected to enhance channel estimation performance with noise mean effects. However, if a multiple input multiple output (MIMO) signal is not correctly detected, it has a problem of deteriorating the channel estimation performance because it estimates a channel with the incorrect information.
The EM method improves channel estimation performance by using the initially-estimated channel response and the received data signal. According to the EM, it is assumed that each signal in a constellation is transmitted with the same probability. The EM estimates a channel by accumulating the values obtained by multiplying the respective channel responses assuming each constellation point as a transmission signal, by a weight value according to a distance between the received signal and the respective constellation points. Here, since all constellation points are used, if the number of the transmission antennas or the number of the neighboring base stations is T and the number of the constellation points is |C|, the EM estimates a |C|T number of channel responses, which is highly complicated. On the assumption that noises added to each data signal are based on the same Gaussian distribution, each data signal is accumulated by the same weight. However, the noise property of the data signals may not be identical in a moment although it is identical statistically. Therefore, it is not preferable to assign the same weight. In the MIMO system, a signal which is received through a single reception antenna is represented by a combination of the channel responses multiplied by the T number of transmission signals, and noises. That is, since T numbers of channel responses are estimated by using the probability value of the data signal which is not orthogonal, the optimal result (i.e., the accurate channel response) is not obtained.